The present invention relates to solar collectors, and particularly to a novel form of evacuated solar collector.
Solar collectors comprise, in general, an absorber blackened to absorb radiation, and enclosed in a housing having a transparent window to allow the entry of solar radiation, the housing also providing some measure of thermal insulation to the absorber. Sometimes several transparent windows with intervening air spaces are used to increase the thermal insulation.
One method known for reducing heat loss from the absorber is to use a selective black surface which has a low emittance "E" at the operating temperature, but a high absorbance ".alpha." to solar radiation. Many methods are described in the literature for producing such surfaces, and emittance values of the order of 0.2 or less are readily obtained commercially. Under these circumstances, the major heat loss from the absorber is by convection, with thermal radiation playing a small role. To reduce convection heat loss, it would be desirable to evacuate the space between the absorber and its nearest envelope. For moderate vacuum, the convection in the air can be suppressed but conduction remains. However, if the vacuum is high so that the free path of the air molecules is large compared with gap dimensions between the absorber and its nearest envelope, then the air conduction loss can also be reduced to a very low value, so that the thermal radiation loss then becomes the major item.
It is not very practical to draw a vacuum on a flat plate solar collector, but if the envelope is cylindrical (and preferably made of glass) the system becomes practical. It has therefore been proposed to construct a solar collector comprising a cylindrical glass tube within which is mounted, approximately in a diametral plane and not touching the tube walls, a selectively blackened absorber in the form of a substantially flat plate with a tubular passage for conducting a heat transfer fluid such as water, oil, or air. Since the rear side of the absorber plate does not "see" the sky, the rear part of the glass tube is silvered to reduce, to very low levels, the residual thermal radiation from the back of the receiver to the glass envelope.
One of the major disadvantages of such a system is the sealing of the absorber tube to the envelope, this requiring a graded metal-glass seal. Another disadvantage is that the rear of the absorber plays only a negative role in the system: it collects no solar heat but is subject to some thermal losses of its own.